I don't know about builder's keys from that era, but modern builder's keys aren't vulnerable to this problem. The builder's key uses a deeper cut on one or more pins, but the owner's key can and should be pretty unique. The only real requirement is that there be at least one cut to a high enough number to allow for a builder's key. There's a good video here: https://www.youtube.com/watch?v=GUCW4OnE6Mc

Maybe they used a master key system instead.